1. Field of the Invention
The present invention relates generally to electrical circuits, and more particularly but not exclusively to pulse width modulation controller integrated circuit chips.
2. Description of the Background Art
A switching mode power supply (SMPS) with flyback topology typically includes a transformer with a primary winding that is controlled by a control transistor. The control transistor is driven by a pulse-width modulation (PMW) controller to induce current on a secondary winding of the transformer. Current induced on the secondary winding charges an output capacitor to generate an output voltage. A supply voltage of the PWM controller may be generated from operating current induced on an auxiliary winding of the transformer.
During startup, the PWM controller may not be fully operational to drive the control transistor to induce current on the auxiliary winding. The SMPS may thus include a high voltage (HV) startup circuit to allow the PWM controller to receive current from an input voltage of the power supply. In a typical implementation, an HV startup circuit includes an HV startup transistor that is turned ON to allow the PWM controller to receive current from the input voltage of the power supply to generate a supply voltage during startup. The HV startup transistor is turned OFF once the supply voltage increases to a level that is sufficient to switch the control transistor and allow operating current to be received from the auxiliary winding.
FIG. 1 shows an example PWM controller integrated circuit (IC) chip 101 with an internal HV startup circuit. Note that the PWM controller IC chip 101 has eight pins. FIG. 2 shows the PWM controller IC chip 101 as employed in an SMPS. As shown in FIG. 2, the PWM controller IC chip 101 includes an HV startup circuit comprising a junction field effect transistor (JFET) being employed as an HV startup transistor. An internal HV startup circuit simplifies usage of the PWM controller IC, but increases the pin count of the PWM controller IC chip by at least two pins: an HV pin for tapping to the primary winding and an NC (no connection) pin to provide high voltage spacing for safety reasons.
FIG. 3 shows an example PWM controller IC chip 151 with an external HV startup transistor 152. Note that the PWM controller IC chip 101 has six pins. FIG. 4 shows the PWM controller IC chip 151 as employed in an SMPS. The PWM controller IC chip 151 controls the HV startup transistor 152 from one pin (pin 3) and receives a feedback signal indicative of the output voltage on another pin (pin 2). Using an external HV startup transistor reduces the pin count of the PWM controller IC chip, but requires additional external circuitry.